Gene Therapy for Adenosine deaminase (ADA) deficiency
Gene Therapy is a treatment where DNA is used to the supplement or alter genes in an organism to treat a disease or condition (1). The disease Adenosine deaminase deficiency (ADA) is a autosomal recessive, metabolic disorder that has been treated with the use of gene therapy by replacing an individuals mutated gene with a functional gene (2). Adenosine Deaminase (ADA) deficiency Adenosine deaminase (ADA) deficiency is a mutation on chromosome 20, which causes a lack of the enzyme adenosine deaminase. Adenosine deaminase is an enzyme that metabolizes purines. It is needed to break down adenosine in food and also needed to turn over nucleic acid in tissue. In humans adenosine deaminase is necessary for the maintenance of the immune system. In Adenosine deaminase deficiency there is a lack of the adenosine deaminase enzyme, which causes an excess accumulation of deoxyadenosine. The excess accumulation of deoxyadenosin causes an increase of S-adenosylhomocysteine in an individual and an accumulation of dATP in all cells in the individual. The increase of S-adenosylhomocysteine blocks the maturation of lymphocytes, which causes a reduction of white blood cells and results in a deficient immune system. The accumulation of dATP in cells stops DNA synthesis so the cells are unable to divide in areas like the thymus where T cells proliferate and develop. Due to the fact that T cells are not able to proliferate and develop the individuals immune system is compromise, this inhibits their ability to fight off infection (1,2,3). ADA deficiency can cause the disease severe combined immunodificiency (SCID), this disease causes neurological, skeletal, hepatic, and behavioral alterations, as well as sensorineural deafness in people affected (4). Gene Therapy A study by Alessandro Aiuti, M.D., Ph.D., et al. investigated the effects of gene therapy on 10 children that presented SCID due to ADA. These children were chosen because they could not have a hematopoietic stem cell transplant from an HLA-identical siblings, which is the treatment of choice. The other possible treatments are the use of alternative donors however there is a high risk of death or lack of engraftment all together, and the administration of polyethylene glycolmodified bovine ADA however this fails to correct the immunodeficiency. Other gene therapies were show to be successful however the patients needed PEG-ADA maintenance after treatment and the ADA- transducer stem cells did not build back up the patients immune system. The treatment in this study included nonmyeloablative conditioning as part of the treatment, which corrected the metabolic and immune defects in the other gene therapy studies (4). Treatment: -Four days before the gene therapy treatment the patients had autologous bone marrow specimens harvested, the mononuclear cells were harvested and the CD34+ cells were purified with immunomagnetic beads. -The CD34+ cells (hematopoietic stem cells) were then stimulated with cytokines and were transduce with the GIADA1 retroviral vector. -The retroviral vector was based on the Moloney murine leukemia virus and carried the human ADA gene. -The nonmyeloablative conditioning included busulfan and was administered 3 days before the gene therapy. -The gene therapy itself included the infusions of CD34+ marrow cells that were transducer with the ADA-containing retroviral vector. This study found that therapy with nonmyeloablative chemotherapy and infusion of hematopoietic stems cells transducer with a retroviral vector with the ADA gene did not have any adverse effects in the 4 year follow up. The replaced hematopoietic stem cells were then able to pass a functional ADA gene on the their progeny. The ten patients in this study have a fully restored immune system (4). Complications with Gene Therapy One possible complication of gene therapy and the use of retroviral vectors is that the gene will be randomly integrated into the incorrect host cell genome and be incorporated into transgene expression (5). This means that the gene can be integrated anywhere into the human genome. This can create many problems, for instance, if the gene becomes inserted near a protooncogene it could lead to tumorgenesis. Another problem with retroviral vector genes is that the expression of the gene being inserted may not last forever, this is because retroviral promoters tend to become inactive over time which means that the "good" gene noloner is active [7.] In the studied that have been conducted using gene therapy the patients could not get a hematopoietic stem cell transplant and did not respond to enzyme replacement therapy with administration of polyethylene glycolmodified bovine ADA so gene therapy was the next best treatment and is a promising one since that alternative is death of the patient. Gene therapy has been successful in most studies it has been used in however because SCID caused by ADA deficiency is a very rare condition and no large scale studies have been conducted, therefore the complications that could result from the gene therapy treatments can not be fully concluded and a outline of the treatment strategies is very limited [6.] References 1. Gene Therapy, Wikipedia, 2013. Link: http://en.wikipedia.org/wiki/Gene_therapy 2. Adenosine deaminase deficiency (ADA), Wikipedia, 2013. Link: http://en.wikipedia.org/wiki/Adenosine_deaminase_deficiency 3. Adenosine deaminase, Wikipedia, 2013. Link: http://en.wikipedia.org/wiki/Adenosine_deaminase 4. Aiuti, A. et al. Gene Therapy for Immunodeficiency Due to Adenosine Deaminase Deficiency. The New England Journal of Medicine. 2009; 360:447-458. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19179314 5.Bonini. C. et al. Safety of retroviral gee marking with a truncated NGF receptor. Nature Medicine. 2003; 9:367-369. Available from: http://www.nature.com/nm/journal/v9/n4/full/nm0403-367.html 6. Gaspar H. How I treat ADA deficiency. blood. 2009; 14:17. Available from: http://bloodjournal.hematologylibrary.org/content/114/17/3524.short 7. Gallo R and Reitz M. Retroviral Vectors and Gene Therapy. Holland-Frei Cancer Medicine. 2003. 6. Available from: http://www.ncbi.nlm.nih.gov/books/NBK13295/